So what does the surface temperature of the North Atlantic Ocean thousands of miles away have to do with the health of spruce trees high in the Colorado Rockies? A fair amount, according to a surprising new University of Colorado Boulder study.

Changes in sea-surface temperatures of the North Atlantic ironically appear to be closely tied to drought conditions in the high northern Colorado mountains, which have turned out to be a key player in triggering a massive spruce beetle outbreak there.

The new study shows that drought is a better predictor of spruce beetle outbreaks in northern Colorado than just temperature, said lead study author Sarah Hart, a CU-Boulder doctoral student in geography. Drought conditions decrease host tree defenses against spruce beetles, which attack inner layers of bark and eventually kill the trees.

Spruce beetles, like their close relatives, mountain pine beetles, are hammering large areas of coniferous forests across the West. While the mountain pine beetle outbreak in the Southern Rocky Mountains is the best known and appears to be the worst in the historical record, the lesser known spruce beetle infestation has the potential to be equally or even more devastating in Colorado, Hart said.

“It was interesting that drought was a better predictor for spruce beetle outbreaks than temperature,” she said. “The study suggests that spruce beetle outbreaks occur when warm and dry conditions cause stress in the host trees.”

The strongest climate correlation to spruce beetle outbreaks was above average annual values for the Atlantic Multi-decadal Oscillation, or AMO, a long-term phenomenon that changes sea-surface temperatures in the North Atlantic. Believed to shift from cool to warm phases roughly every 60 years, positive AMO conditions are linked to warmer and drier conditions over much of North America, including the West.

Geography professor and study co-author Thomas Veblen said the AMO shifted from its cool to warm phase in the 1990s, meaning the climate phenomenon could be contributing to drought conditions in the West into the middle of this century. A 2006 tree-ring study involving CU concluded that the warm phase of AMO also was correlated to increased wildfires in the West.

The new study was published in the journal Ecology. The paper also puts to rest false claims that fire suppression in the West triggers spruce beetle outbreaks.

From 2008 to 2012, roughly 960,000 acres of spruce-fir forests have been affected by spruce beetle invasions in Colorado, while the mountain pine beetle outbreak has so far affected about 3.4 million acres in the state, said Veblen.

The area of high-elevation forests affected by spruce beetles is growing in the West, Hart said. “In 2012, U.S. Forest Service surveys indicated that more area was under attack by spruce beetles than mountain pine beetles in the Southern Rocky Mountains, which includes southern Wyoming, Colorado and northern New Mexico,” she said. “The drought conditions that promote spruce beetle outbreak are expected to continue.”

The CU-Boulder team assembled a long-term record of spruce beetle outbreaks from the northern Front Range to the Grand Mesa in western Colorado using a combination of historical documents and tree ring data from 1650 to 2011. Broad-scale outbreaks were charted by the team from 1843-1860, 1882-1889, 1931-1957 and 2004 to 2010.

In addition to ocean-atmosphere interactions and tree rings, the team looked at past temperatures, precipitation and aridity to better understand the spruce beetle outbreaks.

One big concern about spruce beetle outbreaks is their effects on headwater streams that are important for water resources, said Veblen. “In the short term, trees killed by spruce beetles will lead to less water use by trees and more water discharge into streams. But in the long term, the absence of the trees killed by beetles may lead to less persistence of snow and earlier runoff.”